Around 1970 Luxman company
came with one of its first "high quality" integrated amplifiers on the European market.
This amplifier was a modern design with excelent characteristics.
It had a full
complementary output stage and a differential input stage of the power
amplifier.
It is symmetric supplied and the power supply stage was
generously built for a 2x30W amp.

The SQ503 without the X is an
earlier version that lacks the modern electronic design of the X version, but
looks quite the same.

Luxman SQ 503X

After 40 years + the outside is looking very good.

Before the update the inside looks like this.

Inside the amplifier, top view, new transistors, drivers and
output after the update.

The sound of this amp is a pleasure to listen to,
as the basic schematic diagram design is very good and quite timelessly designed.

Modifications:

This amp needs to be updated as the transistors and the capacitors are outdated.
Many capacitors can be replaced by recent types.
Set the power amp DC offset and
the idling current of the power transistors.
The transistors of the pre stages are often noisy. Use BC550/560C or
equivialent,
you can get the noise figure below the original specs of Luxman.
The drivers are often defective and the power transistors of the output stage
may be obsolete. The final power transistors are TO-66 cased NEC
2SA616 and NEC 2SD155, they are not available anymore.
The ledgendary 2SC1445 TO-66 transistors and PNP equivalent 2SA765, of Sanken
are of
exceptional quality and are the perfect replacement, but hard to get today,
the only plastic replacement is plastic transistors like 2SB595/2SD525 of
Toshiba or equivalent,
you have to change the mounting on the heatsink somewhat when you use plastic
transistors,
and you may find in some cases that the sound quality will not be the same
anymore.
I used BD139/BD140 and BD 292/BD437 of STMicroelectronics and
2SA1694 & 2SC4467
of Sanken as you can see on
the photo:
works great.